Sensors to determine if a seat belt is buckled were first widely used to alert or remind the occupants of a motor vehicle to fasten their seat belts. Recently, airbags have been used in vehicles to enhance and complement the protection provided by seat belts. This combination of seat belts and airbags can be optimized if it can be determined that the seat belt is actually in use. If the seat belt is buckles then the deployment of an airbag can be optimized to protect a seat-belted occupant. Accurate sensing information of seat-belt latch status is far more important when used as part of the decision logic and deployment strategy of an airbag system then when used simply as part of a seat belt fastening reminder system.
A typical seat belt latch indicating system such as might be used in combination with an airbag deployment system, is a mechanical switch within the buckle of a seat belt combined with a resistor network. The simplest resistor network has a first resistor connected in series with the switch and a second resistor connected in parallel with the switch. Thus the resistance of the switch and the resistor network changes when the switch is closed. But the switch, because of the resistor connected in parallel across the opens leads of the switch, can be positively monitored when opened. Thus if resistance falls to zero or has a resistance value substantially different from the two designed states of the switch and resistor network, it is clear that the seat belt latch detector is broken.
The installed seat belt environment can be a hostile one. The switch may be called upon to operate even when dirty, wet, or even when coated with residue from spilled beverages. Further, the use of seat belt latch sensors to sound reminder chimes or a buzzer is sometimes perceived as annoying, and can lead those resistant to wearing safety belts to disable a seat belt latch indicator by causing the buckle to latch without engaging the hasps of the belt. Where seat belt latching information is being used by an airbag deployment logic, the folly of disabling safety equipment can prevent the deployment logic from making the best possible decision by providing false information to the logic.
What is needed is a seat belt latch indicator which is both durable and better able to monitor latch status.